The present invention relates to a laser processing apparatus, and more particularly, to a laser processing apparatus having an improved optical system for transmitting a laser beam from a laser oscillator to a workpiece.
FIG. 14 is a diagram of an outline of a conventional laser processing apparatus. This diagram has been simplified and merely illustrates the arrangement of the laser processing apparatus. As shown in the figure, a laser beam 2 emitted from a laser oscillator 1 is converted into a converged beam 3 by a converging lens 4, and this converged beam 3 is converged to a focus 7 on a workpiece 5. The workpiece 5 is set on a worktable 6. The position of the worktable 6 is controlled in such a manner that the finished workpiece 5 has a desired shape.
As shown in FIG. 14, the laser beam 2 emitted from the laser oscillator 1 has a divergence angle of 2.theta..
If the focal distance of the converging lens 4 is f, the diameter of the spot on the focus 7 obtained when the laser beam 2 is converged is 2f.theta.. To improve the machining accuracy, however, the spot diameter should be minimized, and thus the machining accuracy can be improved by reducing the focal distance f or the divergence angle .theta..
If the focal distance f is shortened, however, the spherical aberration is increased, and the focal depth is lessened. In general, therefore, it is not advisable to shorten the focal distance f.
Accordingly, an optimum reduction of the spot diameter is obtained by narrowing the divergence angle .theta., and various methods are used to this end. The following is a description of those methods.
The divergence angle .theta. is generally given by EQU .theta.=2.lambda.Cmn/.pi.D, (1)
where .lambda. is the wavelength, Cmn is a constant dependent on the mode, and D is the oscillator output aperture diameter.
A first method used to narrow the divergence angle is that in which the TEM00 mode is used to adjust Cmn to a minimum value 1. This method has a drawback in that the output is reduced.
According to a second method, the oscillator output aperture diameter D is enlarged. This method is inconsistent with the aforesaid requirement for the use of the TEM00 mode.
A third method uses a beam expander to eliminate the drawbacks of the foregoing two methods. FIG. 15 is a diagram illustrating the use of a beam expander as a means of reducing the spot diameter.
According to this third method, a laser beam 2 in the TEM00 mode and having a small diameter D1 is emitted from a laser oscillator 1. This laser beam 2 is converted into a beam having a large diameter D2 and a narrow divergence angle, using a combination of two lenses, i.e., by using a beam expander composed of a concave lens 8 and a convex lens 9. This beam expander converts the laser beam having the small diameter D1 into a laser beam having the large diameter D2. It further, reduces a divergence angle 2.theta.1 to a divergence angle 2.theta.2.
Since the divergence angle .theta. is inversely proportional to the laser beam diameter D, as is evident from equation (1), the following equation can be obtained if "1" and "2" are suffixed to the respective constants of the individual laser beams before and after the passage through the beam expander. EQU .theta.2=(D1/D2).theta.1. (2)
Even though the focal distance f of the converging lens 10 is long, the spot diameter at the focus can be made small enough by narrowing the divergence angle .theta.2 of the laser beam after the passage thereof through the beam expander.
This method, however, has the following drawbacks. First, the beam expander is expensive, and second, the passage of the laser beam through the beam expander causes an absorption of the laser output. That is, the use of the beam expander leads to an increase in the number of lenses used, which results in increased costs and the need for an augmented laser output absorption.